Extended Oxygen Delignification with the OxyTrac Process, 1999 Pulping Conference Proceedings

Monica Bokstršm

Valmet Fiber Technology

Oxygen delignification at medium pulp consistency combined with thorough washing is a well established process technology for chemical pulps. The first full scale commercial oxygen delignification system was delivered by Sunds Defibrator in the beginning of the 70Õs.

Installation of an oxygen delignification system means that the bleaching chemical consumption and the environmental impact from the bleach plant can be reduced. Oxygen delignification is more selective than the residual delignification in cooking. Lately, there has therefore been a growing interest to combine an increase of the kappa number after cooking with extended oxygen delignification. In this way it is possible to increase the yield of bleached pulp.

Until recently the general opinion has been that the maximum delignification possible for softwood pulp is about 50% in oxygen delignification with maintained pulp strength properties. There was a very good ground for this thought as the possible delignification by oxygen reactions is limited to certain groups of the residual lignin after cooking.

The mills that installed the oxygen delignification systems generally had very good experience of it as it is easy to operate and it levels out variations in digester kappa numbers before bleaching. The goals with the installations, as reduced chemical consumptions and reduced effluent load from the bleach plant, were fulfilled.

The interest to improve pulp yield and still to maintain or even reduce the chemical consumption and effluent load from the bleach plant has increased during the last years. Many studies and attempts to increase the delignification by oxygen were started. Several concepts for extended delignification have been developed and some of these are in operation.

One requirement for extended oxygen delignification is that the selectivity of the delignification is better compared to the conventional delignification. The most simple way to extend the delignification somewhat is to increase the residence time in the reactor and to reduce the temperature.

However, most systems for extended oxygen delignification are two reactor systems, with or without interstage washing. With interstage washing dissolved organics are removed before the final delignification and fresh chemicals are charged to the pulp before the second stage, which gives some improvements of the selectivity. The drawback is the high cost for this system.

The first systems for two stage delignification without interstage washing were mainly operated to make it possible to split the chemical charges between the reactors.

The two stage system developed by Sunds Defibrator, the OxyTrac TM , is based on the kinetics of oxygen delignification as well as on our practical experience of running one and two stage delignification at different conditions.